All of the so-called natural cresylic acid feedstocks (containing phenol, cresols, ethylphenols, xylenols and C.sub.9 phenolics) are derived from either coal or petroleum processing. Petroleum refinery "spent caustic" is derived via extraction (with aqueous sodium hydroxide) of phenols and thiophenols (along with traces of neutral oils and tar bases) from oil refinery distillate products. Neutral oil (as used throughout this text) is a class of organic impurities (indigenous to natural cresylic acid feedstocks), each one of which is neither an acid nor a base, such as indenes, indans, ketones and naphthalenes. Tar bases (as used throughout this text) constitute a class of organic impurities (indigenous to natural cresylic acid feedstock), each member of which is a nitrogen-containing compound which behaves as a base, such as pyridine, alkylpyridine, aniline and alkylaniline.
Processing of spent caustic into purified cresylic acid usually consists of two or three steps: one for separation of sulfur compounds from phenols, a second step (sometimes omitted) for separation of neutral oil substances and tar bases via a steam distillation step to steam them from the caustic solution and a final (third) step (always included) to neutralize the sodium cresylate with an acid or acid gas in order to "spring" the free phenols (liberate them as an oil phase). Such purified cresylic acid mixtures, after drying and depitching, are either sold as such, or they are fractionated into various products, such as phenol, ortho-cresol, meta/para-cresol, 2,4/2,5-xylenol and high boiling xylenol/ethylphenol/C.sub.9 phenol mixtures.
Such a process provides a raw cresylic acid feedstock suitable for the subject process, provided the steaming step for removal of tar bases and neutral oil is either eliminated or reduced in severity. (As used in the text and claims, "raw" refers to dried and depitched material from which tar bases and neutral oils have not been removed.)
Coal-derived feedstocks come from coal process technologies, such as coking, gasification or other coal devolatilization processes. All such coal technologies yield at least two condensate streams: 1) coal tar oil (to be discussed first) and 2) phenolic-rich condensate water (to be discussed later). Both of these materials result from cooling gaseous devolatilization products from heating of coal; they are condensates.
Coal tar oil is useful as a feedstock for manufacturing cresylic acid (among other products). A typical coal tar oil contains moisture (2% to 5%), naphtha (5% to 10%), middle oil distillate, also known as carbolic oil distillate or tar oil distillate (30% to 40%) and a distillation bottoms product somewhat resembling creosote, the balance.
The middle oil (or carbolic oil) distillate fraction (from coal tar oil), which is useful for manufacturing cresylic acid, is made up of phenolics (30% to 40%), neutral hydrocarbons (approximately 60% to 70%) and tar bases (1% to 3%). This material can be processed into either finished cresylic acid mixtures or raw, impure cresylic acid mixtures by any of a number of techniques.
The oldest of all these technologies was developed in the late 1800's. This process utilized a sodium hydroxide solution to extract phenols from neutral hydrocarbon oils. The caustic extract was decanted from the majority of the neutral hydrocarbons (which formed a separate phase). Inevitably some neutral oil substances and the majority of the tar bases present in the middle oil were extracted into the caustic solution. The caustic solution of phenols was then heated in order to steam distill these two classes of impurities, neutral oils and tar bases. In practice, this steaming step has never been pushed hard enough to remove all of these impurities. The purified aqueous sodium phenolate solution was then reacted with carbon dioxide by sparging stack gas into the solution. The carbon dioxide reacted with the sodium salt of the phenols to form free cresylic acid (an oil phase) and a sodium carbonate solution (an aqueous phase). The aqueous phase was separated from the sprung phenols by decantation, and then reacted with quicklime to regenerate sodium hydroxide for recycle. The sprung phenols were then dried via distillation of moisture, and then depitched via distillation to separate the purified cresylic acid from any pitch-like substances formed during the steam distillation step.
The product of this process is useful as finished cresylic acid, or it can be fractionated into finished cresylic acid distillate products.
If the steaming step to remove tar bases and residual neutral oil were either eliminated or reduced in severity, the sprung, dried and depitched raw cresylic acid would be a suitable feedstock for the subject process.
This oldest technology has some significant disadvantages. It is bulky and energy intensive, and it creates waste streams and materials which are hazardous and difficult to remediate. For this reason, chemists and engineers went on to develop solvent extraction technologies to separate neutral oil from phenolic substances. These solvent technologies were inferior in at least one regard to the earliest technology, since the latter separated both neutral oil and tar bases.
Many of the numerous solvent extraction technologies (developed since this earliest technique) employed the use of a single solvent to extract phenols from a neutral oil (middle oil) matrix. Aqueous solutions of a number of solvents, such as glycols, ethanolamine, ammonia, acetic acid, ethylamine, sodium salicylate and methanol, were employed. Hot water was another single-solvent approach which was used. The solvents were usually removed from the extract mixture by way of distillation and recycled to the front end of the process. All of these technologies were faced with the problem of obtaining adequate cresylic acid product purity. The phenols which were isolated via these technologies contained significant amounts of residual neutral oil contaminants and tar bases. This problem was especially prevalent when a high yield (degree of recovery) was obtained: with high product yields it was inevitable that large amounts of neutral oil and tar base impurities would be found in the phenolic extract (solvent) phase. Any of these single-solvent processes would provide a raw cresylic acid ready for fractionation into impure distillate fractions suitable for the subject process. Significant amounts of such impurities pose no problem for said process.
Because of the purity vs. yield problems of the single-solvent techniques, chemists and engineers went on to develop dual-solvent (fractional countercurrent extraction) techniques. These technologies provided the ability to obtain high purity and high yield at once, by using a pair of solvents; one, a polar solvent to dissolve the phenols, and another, a non-polar solvent to dissolve the neutral oil impurities. Examples of polar solvents are aqueous solutions of a number of solvents, including methanol, ammonia, acetamide, acetic acid, ethanol, monoethylamine, sodium salts of sulfonic acids, etc.; and examples of non-polar solvents include hexane, heptane, petroleum ether, diesel and various non-aromatic naphthas. Distillation techniques were used to regenerate both polar and non-polar solvents for recycle.
Like the single solvent techniques, none of these technologies could achieve a good separation of tar bases from phenols; only neutral oil substances were separated. The cresylic acid materials derived from dual solvent processing required another treatment step to remove tar bases prior to sale as a mixed cresylic acid finished product or prior to fractionation for finished distillate products.
Any of these dual solvent technologies could be used to obtain a raw cresylic acid which would be suitable for the subject invention. Indeed, any of these processes could be operated (for purposes of the present invention) in a most economical fashion (such that neutral oil substances would not be thoroughly removed), since the instant technology can easily handle significant amounts of these substances.
Other types of dual solvent-like processes which were developed were the Phenoraffin process and others similar to it. In this technology, saturated aqueous sodium cresylate served as the polar solvent, and toluene (or the like) in a second extraction step was comparable to the non-polar solvent. The sodium cresylate solution, which became "oversaturated" with dissolved free phenols, was boiled in order to steam distill toluene, and some of the neutral oil not removed in the primary extraction steps. A solvent, such as diisopropyl ether, was then used to extract the free phenols from the sodium cresylate solution, thus regenerating it for recycle to the front end of the process. The isopropyl ether solution of the phenols was then extracted with aqueous sulfuric acid to remove tar bases, and finally the ether was distilled for recycle, leaving the purified phenols (ready for drying and depitching) as a still-bottoms product.
This process, if 1) operated in a most economical fashion (such that it would not remove all of the neutral oil), and if 2) the sulfuric acid step were eliminated, would yield a raw cresylic acid mixture suitable for the subject process.
The East Germans developed technologies based upon the use of calcium hydroxide for purification of cresol mixtures. Cresols were mixed with water, heated, and then reacted with calcium hydroxide to form the calcium cresolate salt (water soluble). The Leuna Werke plant in East Germany patented such a process in 1962 for removal of neutral oil from cresols (British Patent No. 895,119). This aqueous salt was filtered and then further diluted with water to cause neutral oils to be "sprung" from the solution as a black oily liquid (neutral oil substances are less soluble in dilute aqueous calcium cresolate solution than in a more concentrated solution). The product cresolate solution was then acidified with hydrochloric acid in order to spring the cresols, making them ready for drying and depitching.
In order for this process to be implemented in a more economical fashion, the amount of water used to dilute the calcium cresolate solution (to spring the neutral oil, the black oily liquid) could be diminished (or eliminated altogether). In this way, the product (cresols) would be less pure, but would be suitable as raw cresols for the subject process.
Since few of the foregoing technologies were capable of removing tar bases, a number of tar base removal processes were developed. The most elementary (and probably most common) was a process which could be called the acid flash distillation process. Sulfuric (or less commonly phosphoric) acid was added to cresylic acid (or a fraction thereof in some few instances), and this mixture was distilled in either a batch still or a continuous flash drum. The tar bases were rendered non-volatile via salt formation with the sulfuric acid, and were collected as a still-bottoms product. The overhead product was obtained as a very nearly tar-base-free material.
The Leuna Werke plant in East Germany, which (in 1984) patented a glycol-based extractive distillation process to remove guaiacol from meta/para-cresol, developed a unique version of the acid flash process, also patented in 1984. They removed tar bases from phenols by adding sulfuric acid to the fractionation tower while it was being used to fractionate the distillate products (such as meta/para-cresol) which can be manufactured from cresylic acid.
Another variation of the acid flash process was developed in Poland. It was very similar to the continuous version of the traditional acid flash method, except that water was used in the distillation to limit the temperature of the process (and thus limit the degradation of phenols to tar-like residue).
An earlier West German patent for removal of tar bases described a process wherein a cresylic acid distillate fraction was mixed with a strong acid and some toluene (or other aromatic) and cooked for a time. This was followed by a distillation wherein the overhead product, cresylic acid, was treated with activated alumina. The tar base/toluene condensation products were removed as a still-bottoms product.
In an East Indian dissociative extraction process, cresylic acid was dissolved in a solvent, such as chlorobenzene or 2-ethylhexanol, and this mixture was extracted in a countercurrent fashion with aqueous hydrochloric acid. The solvent was distilled from the phenols and recycled. Tar bases were removed from the extraction column as the hydrochloride salts.
Patents have also been granted for the use of strongly acidic cation exchange resins to remove tar bases from cresylic acid. Similarly, the Japanese have patented the use of acidic clays to remove traces of tar bases from cresylic acid.
All of the above tar base removal techniques were developed as an adjunct step to supplement a neutral oil removal step.
All of the above descriptions of processes for removal of neutral oil and tar bases were cited (for purposes of this document) in the context of tar oil (or middle oil) distillate processing.
As noted earlier, the condensate water stream which results from cooling of gaseous coal devolatilization products is rich in dissolved phenolics. Such aqueous condensate (gas liquor) streams are typically subjected to a solvent extraction process in order to extract the phenols from the water, as is well known in the art. The solvent is then distilled from the phenols and recycled. Typical of these process technologies is the Phenosolvan process, which utilizes diisopropyl ether or a similar solvent as the extraction solvent.
A typical Phenosolvan extract mixture of phenols (known as crude phenol) usually contains approximately 60 to 75% monohydric phenols (cresylic acid) and also contains dihydric phenols (such as catechol) and pitch (20% to 25%), neutral oil (1% to 4%), tar bases (1% to 3%) and water (2% to 6%).
Processing of phenolic extracts derived from such wastewater should first include a process step for separation of the monohydric phenols from the dihydric phenols and pitch. This separation is typically accomplished by way of a distillation (depitching) step. Initial processing should also include a drying step for separation of water via distillation. The monohydric phenols cut, thus isolated, is a fairly pure material compared to middle oil distillate; it contains only 1.5 to 4% tar bases and 1.5 to 5% neutral oil. Such material 1) can be processed via any of the earlier-described neutral oil and tar base removal techniques designed for the far less pure tar oil distillate mixtures, or 2) can be blended with tar oil distillate and processed by any of these same earlier-described techniques.
The cresylic acid industry in the past focused almost exclusively upon purification technologies which treat a broad boiling-range mixture of phenols (C.sub.6 through C.sub.9) in order to render such mixtures free of the two principal classes of contaminants indigenous to natural cresylic acid mixtures (both coal and petroleum-derived): neutral oil and tar bases. Once free of these impurities, cresylic acid mixtures were either sold as such, or fractionated into distillate products and then sold. At times such distillate products were processed for even further upgrading.
Such further upgrading (of the individual distillate fractions from natural cresylic acid mixtures) almost exclusively dealt with the topic of further separations of phenolic substances from one another (which cannot be accomplished via ordinary fractionation techniques). Feedstocks for such phenolic separation technologies have been distillate fractions derived via fractionation of tar-base- and neutral-oil-free phenolic mixtures.
Consolidation Coal (later known as Pitt-Consol) patented glycol extractive distillation for removal of 2,6-xylenol, an unwanted phenolic substance, from m/p-cresol (a discrete distillate fraction derived from cresylic acid). This process was described in their 1967 patent (U.S. Pat. No. 3,331,755) granted to Martin Neuworth, assigned to Consolidation Coal Co.
Pitt-Consol used the acid flash process for removal of tar bases from a wide boiling range mixture of phenols prior to fractionation to isolate the m/p-cresol distillate fraction. Earlier, under the name of Consolidation Coal, they used the dual solvent technology described in U.S. Pat. Nos. 2,666,796 and 3,079,326 for removal of neutral oil from a coal-derived middle oil distillate (a wide boiling range mixture). (It should be noted that Pitt-Consol later used a similar dual solvent technology to process oil refinery spent caustic-derived cresylic acid; see U.S. Pat. Nos. 2,767,220 and 2,789,145.) The m/p-cresol described in U.S. Pat. No. 3,331,755 was thus tar-base and neutral-oil free.
Extractive distillation of a cresylic acid fraction with glycols was also described in East German Patent No. DD 204,474, granted to the Leuna Werke plant for removal of guaiacol from m/p-cresol via glycol extractive distillation. This patent also described the partial removal of ortho-ethylphenol from m/p-cresol. As noted earlier, 1) Leuna Werke patented a calcium salt technique for removal of neutral oil from cresols, and 2) they also patented the use of sulfuric acid to remove tar bases in-situ during fractionation. Based on these patents, the m/p-cresol described in East German Patent No. DD 204,474 is thus tar-base and neutral-oil free.